1. Technical Field
The present invention relates to trailer hitch structures that are capable of efficient absorption of an impact load of a rear end collision.
2. Related Art
A conventional trailer hitch is typically attached to the rear of a vehicle, such as an automobile, to pull a camping trailer, a trailer carrying a small boat, or the like. In most cases, the trailer hitch is connected directly with highly rigid rear side frames (also called “rear side members”) that are provided on either side of a vehicle widthwise direction and extend toward the rear of the vehicle body.
Thus, the trailer hitch includes fixed plates on either side that are fixed to rear end surfaces of the rear side frames respectively. The fixed plates are connected to each other by a cross member, and a hitch support arm for supporting a hitch ball is fixed inside the cross member.
In a state in which the fixed plates are fixed to the rear end surfaces of the rear side frames, this hitch support arm is offset downward with respect to the rear side frames and protrudes towards the rear of the vehicle.
Due to this configuration, at a rear end collision of the vehicle, an impact load is applied first to the hitch support arm. Because this hitch support arm is offset downward with respect to the rear side frames, this impact load generates a large bending moment in the fixed plates via the hitch support arm to rotate the fixed plates downward. Due to this moment load, each rear side frame becomes bent and deformed easily at a point in front of the fixed plates. As a result, the rear side frames are prevented from efficiently absorbing the impact.
As a way to deal with this problem, Japanese Unexamined Patent Application Publication (JP-A) No. 2009-262660, for instance, discloses a technique in which a fold promoting member is fixedly installed to a rear end of rear side frames, and a trailer hitch that is fixed to a rear end surface of each rear side frame is provided with an abutting portion that is opposed to a rear of the fold promoting member with a predetermined space therebetween, and a fragile portion is formed in the middle of the side frame by means of beads.
According to the technique disclosed in JP-A No. 2009-262660, at a rear end collision, first the reaction of the fold promoting member bends and deforms the rear side frame at a point closer to the rear end side than the fragile portion, to absorb the impact, and then bends and deforms the rear side frame at a point closer to the front than the fragile portion, to absorb the impact.
According to the technique disclosed in JP-A No. 2009-262660, the fragile portion is formed in the middle of the rear side frame, and the rear side frame is bent and deformed twice, in front of and behind the fragile portion, thereby absorbing the impact of a collision.
However, because the trailer hitch is offset downward with respect to the rear side frame, a longitudinal moment is generated in the trailer hitch at the time of the rear end collision, and consequently the rear side frame is bent and deformed with respect to the fragile portion by this moment load. However, simply bending and deforming the center of the rear side frame is not enough to efficiently absorb the impact load of the rear end collision.
Moreover, because the technique of JP-A No. 2009-262660 is structured to absorb the impact load of the rear end collision by bending and deforming the rear side frames twice in front of and behind the fragile portion, the rear side frame needs to ensure therein a relatively long region to be bent and deformed.
However, in a vehicle, such as an engine/electric hybrid vehicle and an electric vehicle, the center of gravity lowered by having a number of secondary batteries such as lithium ion batteries and nickel hydride batteries installed under the rear floor panel between rear side frames to secure driving performance and the like. Such batteries are solid and relatively heavy parts among the parts configuring the vehicle. Accordingly, a relatively long space for absorbing the impact of a rear end collision cannot be secured in each rear side frame in a manner described in JP-A No. 2009-262660.